2003 Town Car A/C comp. won't turn-system's full!

[Canadastang]

Just changed the a/c accumulator on my '03 Town Car - the bottom of it rusted out, super common issue.

Had a reputable shop vacuum and recharge the system, they said the system held vacuum for an hour and there were no leaks...but the a/c compressor wouldn't engage.

The clutch will engage if it's jumpered, but won't engage on its own. The system does work great when it's forced to run.

I did some reading and someone on the crownvic forums said the EATC headunit sometimes needs to be reset by disconnecting the battery, which I did - and the a/c worked! But then when I turned it off and back on, it stopped working.

The clutch gap is about the size of a business card, I can get a better measurement with feeler gauges tomorrow.

Anyways, since I do most of my work myself, I didn't pay the shop to dig further into this issue, but a/c systems are kind of foreign to me, this is really the first time I've worked on a/c systems in a car, but I'm no stranger to powertrain, brakes, suspension and electrical issues, so maybe some experienced BITOG'ers here can steer me in the right direction!

 

[Colt45ws]

Okay, there are a couple of sensors. There is a evaporator temperature sensor on the HVAC box for the PCM. Its between the evaporator and the heater hoses.
Ill look up and see if I can find a ohms to temperature chart but since it happened after repair I would be looking at disconnected wiring.
There is a low pressure switch on the accumulator itself. It should be shorted unless the pressure is low.
Then while the car is running, check for voltage at the RD/BK wire that is going to the pressure switch.
There is also a pressure sensor for the PCM on the high side. Its on the hard lines down by the compressor. Again, I would think it didnt just fail and you should be able to make sure its unplugged.

Do you have the ability to look at PIDs in the PCM (Like Forscan?)

 

[Colt45ws]

Here are the schematics.

The EATC supplies + to the low pressure switch which is then seen by the PCM as long as the switch is closed. (Pressure is high enough)

Wiring of the high pressure sensor and Evap discharge sensor

Wiring of the Relay, controlled by the PCM.

 

[kc8adu]

i dont recall what the system does if the clockspring fails.since it senses the resistance to control the system if it fails it could have defaulted to a high temp and the system stays off.at least that is what i am seeing in the above diagram.
and the clockspring is a high failure part.
if resetting the system brought it back temporarily this is most likely.

 

[Chris142]

Did you jump the wires at the low pressure switch?

 

[mk378]

Swap out the compressor relay. There is usually another relay of the same type in the box.

See if power appears on the compressor wire when it should be on. If there is voltage to the compressor but it does not engage, that would mean a clutch issue.

Confirm that voltage appears on the pink/lb wire of the pressure switch when A/C is turned on and goes away when turned off. That shows the EATC and switch are OK.

Upon receiving the signal from the EATC via the pressure switch, the PCM reads the pressure transducer and evaporator temperature sensor. If both are OK it supplies ground to the relay coil pulling in the relay to engage the compressor.

 

[Colt45ws]

Originally Posted By: kc8adu
i dont recall what the system does if the clockspring fails.since it senses the resistance to control the system if it fails it could have defaulted to a high temp and the system stays off.at least that is what i am seeing in the above diagram.
and the clockspring is a high failure part.
if resetting the system brought it back temporarily this is most likely.


If it was failing and got spurious signaling then yeah it might change settings randomly, but you could just cut the BN/LB wire at the back of the EATC and it would operate fine.
The EATC in my CVPI came from a car with steering wheel buttons and when I converted it I just left it open and I have no issue.
Tbh since it happened right after the accumulator replacement, Im going to guess the low pressure switch is bad, as that was supplied when I bought my accumulator. I would jumper the two wires going to the switch and see if A/C operates. It might even cycle OK since the evap temp sensor will call for the A/C to cycle off if the core gets too cold.

 

[Errtt]

I had a truck and the system wouldn't engage the compressor (clutch).
I was like, oh no! and figured I'd start easy and see where it ends.
Lucked out right away and found the low pressure switch went bad. AC worked as should when I jumped the connector.
I felt lucky that's all it was in my case and didn't spend a ton of time.

 

[mk378]

You can R&R the low pressure switch with the system charged. There is a valve in the fitting that closes as the switch is unscrewed. This also means the switch needs to be screwed on all the way for it to see pressure.

 

[Canadastang]

Thanks to everyone for the replies and wiring diagrams.

I took a stab at it today and now I'm stumped.

I jumped the low pressure switch, but the clutch did not engage.

There was 12v present at the red and black wire at the low pressure switch with the ignition on, whether the a/c was switched on or not. No voltage present at the pink and blue wire, regardless of a/c being switched on.

Hooked up a test light to the a/c clutch wiring - no voltage present when a/c commanded on.

Ran a self test on the EATC - no trouble codes.

It just doesn't make sense considering the a/c engaged once yesterday after I disconnected the battery.

So I'm thinking it's gotta be either a/c relay or bad EATC control unit

Thanks in advance everyone.

 

[Colt45ws]

If there is voltage at the RD/BK then the EATC is calling for A/C and its fine.
Did you check PK/LB while it was plugged into the pressure switch? Thats the wire that goes to the PCM and tells it to turn on the compressor.
If you plug it into the pressure switch, turn on the A/C you should have voltage at both the RD/BK and PK/LB. You already said RD/BK is good so we need to make sure its going to the computer.
There should be other relays in the box that are the same. should be able to swap it with a different one and see if it is the relay
Do you have the ability to read PCM data like Forscan?

 

[Canadastang]

No, I just stuck a test light into the harness with it unplugged.

I was going to try swapping a relay but was having a [censored] of a time getting it out of the box...is there a trick to them?

It didn't want to just pull straight out like the other relays. Weird.

I have some limited PCM scanning ability, well basically just a glorified code reader than can check some PID's, whether it can check HVAC stuff I'm not sure.

 

[Colt45ws]

Yeah, they get stuck in there real good and there is no room to get a good hold of them so yeah, I can see that as an issue.
Also, should probably stick with a multimeter; the EATC isnt really designed to drive a bulb off its output. Its probably just got a small transistor, would only be expecting the PCM load; maybe like a 10k ohm load or something.

 

[Canadastang]

Well I'm even more stumped now.

Swapped the horn relay with the a/c relay. Horn stills works, but a/c does not.

When I test the low pressure switch wiring with it plugged in, the red wire still has 12v with the a/c commanded on, but the pink wire does not.

But if I jump the low pressure switch, the a/c still doesn't engage.

Any ideas?

 

[mk378]

Like someone else said use a voltmeter NOT a test light on the control system wiring.

If you don't have the same voltage on both sides of the switch it is not a closed circuit like it should be. Unplug the switch and measure ohms between the pins to see if the switch itself is closed. The plug could be bad.

 

[Colt45ws]

Yes, might have a bad connection on the pink wire to the low pressure switch. Ohm out the low pressure switch and if its very low resistance (a couple of ohms at most) then I would be looking at the plug for corrosion or maybe the pin pushed back into the plug and is not making contact.

 

[Canadastang]

I ohm'd the low pressure switch - it's fine, about 1 ohm.

Oddly enough, I tried a multimeter on the red wire, and it's got a constant 12 volts with the ignition on, regardless of whether the a/c is on or not. Yes, I turned the entire EATC off, and it's still got 12 volts at the red wire.

I tried jumping the plug at the low pressure switch with it still plugged in and the compressor still didn't run. Connector looks to be in good shape, wires are tight and no corrosion.

So now I'm thinking either an EATC issue or PCM issue (unlikely)

 

[Colt45ws]

That is very odd. Ill check my car in a bit to see how it behaves with the input to the pressure switch and the EATC in various states. If it was stuck on then your problem would be the opposite; A/C that runs all the time.

 

[mk378]

Some of these systems are negative logic -- voltage at or near ground means on, 5 volts or battery voltage for off. A service manual would need to be consulted.

Voltage not changing when you turn A/C on and off at control panel suggests the EATC is not working.

 

[punisher]

If you have a scan tool that reads data/PIDS function this isn't a tough diag. Monitor "AC DEMAND", and keep an eye on the low pressure, high pressure cutout status, evap temp. At AC relay-Verify power at pin 2, ground at pin 1 if AC demand is present and all sensors are happy. EATC self test codes?